1. Field of the Invention
This invention relates to an apparatus for analyzing air/fuel ratio sensor characteristics.
2. Description of the Related Art
An exhaust purifying system that uses a three-way catalyst is often used for the internal combustion engine of a vehicle such as an automobile, to greatly reduce the harmful components discharged in the exhaust. The basic function of a three-way catalyst is to efficiently purify all three principal harmful components contained in comparatively high concentrations in such exhaust gases, which are nitrogen oxides (NO.sub.x), carbon monoxide (CO), and hydrocarbons (HC). However, the purifying efficiencies of the three-way catalyst are greatly dependent upon the air/fuel ratio (air ratio .lambda.) of the exhaust, as shown in FIG. 22. In other words, when the air/fuel ratio of the exhaust is fuel-rich (hereinafter abbreviated to "rich"), the purifying efficiencies with respect to carbon monoxide (CO) and hydrocarbons (HC) drop; conversely, when it is fuel-lean (hereinafter abbreviated to "lean"), the purifying efficiency with respect to nitrogen oxides (NO.sub.x) drop dramatically. As a result, the range within which the purifying efficiency of the three-way catalyst with respect to each of the three principal harmful components is high is limited to an extremely narrow range (of within 1%) in the vicinity of the stoichiometric air/fuel ratio. This means that the (time-averaged) air/fuel ratio of the exhaust must be controlled precisely at the stoichiometric air/fuel ratio in order to make the most of the intrinsic function (purifying efficiency with respect to the three harmful components) of the three-way catalyst.
If air/fuel ratio control is provided by an open-loop control method that does not use an air/fuel ratio sensor, it has been determined that the accuracy of the air/fuel ratio is mainly dependent on the accuracies of an air flowmeter and fuel flowmeter. To ensure highly accurate air/fuel ratio control with this method, the two flowmeters are required to have extremely high measuring accuracies, or rather, extremely high manufacturing accuracies. However, it is not easy to satisfy the demands for the above described preferred precise (to within 1%) air/fuel ratio control with today's levels of manufacturing technology, even if the manufacturing accuracy of these flowmeters could be raised to the maximum.
In such a case, it is usual to employ a stoichiometric air/fuel ratio sensor (hereinafter called "O2 sensor") for the exhaust to precisely measure a deviation from the stoichiometric air/fuel ratio, then apply precise stoichiometric air/fuel ratio control by a closed-loop control method. This closed-loop control method differs from the above described open-loop control method in that the accuracy of the air/fuel ratio control that provides depends mainly on the accuracy with which the air/fuel ratio sensor detects the air/fuel ratio; the accuracies of the above two flowmeters have only a secondary effect. In other words, if there is any deviation in the air/fuel ratio detection characteristic of the air/fuel ratio sensor when the closed-loop control method is used, it will have a direct effect on errors in air/fuel ratio control. Therefore, the air/fuel ratio detection accuracy of a stoichiometric air/fuel ratio sensor is an important factor affecting the performance of this three-way catalyst type of exhaust purifying system. That is why it is essential during the fabrication of a stoichiometric air/fuel ratio sensor to perform a complete check on the accuracy with which it detects the air/fuel ratio.
Apparatuses for checking the air/fuel ratio detection accuracy of stoichiometric air/fuel ratio (O.sub.2) sensors are known, such as the O.sub.2 sensor evaluation apparatuses disclosed in Japanese Patent Nos. 1417297 and 1417298. With such an evaluation apparatus, the characteristics of a stoichiometric air/fuel ratio (O.sub.2) sensor can be evaluated to an accuracy on the order of 1%, converted into an air/fuel ratio detection accuracy. Other related patents are listed in Table 1.
TABLE 1 __________________________________________________________________________ Filing Date Content Applicant (date of Test Evaluated Object of (Author) receipt) Number Conditions Value Evaluation __________________________________________________________________________ NGK SPARK Dec. 10, 1987 JP-A-63-314450 Combustion D.sub.R Control air/ PLUG CO LTD exhaust fuel ratio June 22, 1988 Journal of the Combustion exhaust Society of Automotive and bottled gases Engineers of Japan vol. 42, No. 11 HGK INSULATORS Nov. 14, 1988 JP-A-2-132363 Combustion Response Control air/ LTD exhaust time fuel ratio Aug. 5, 1988 JP-A-2-45750 Engine D.sub.R Emissions exhaust MITSUBISHI May 31, 1988 JP-A-1-302155 Bottled Static & MOTORS CORP gases Response characteristics MAZDA MOTOR Dec. 14, 1987 JP-A-1-155257 Combustion Response Emissions CORP exhaust time June 10, 1987 JP-A-63-308554 Combustion Response Emissions exhaust time NISSAN MOTOR Aug. 4, 1981 JP-A-58-22945 Combustion D.sub.R Control air/ CO LTD exhuast fuel ratio June 18, 1981 JP-A-57-208443 Combustion or Response Control engine exhaust waveform frequency Apr. 1, 1981 JP-A-57-163863 Combustion Response exhaust time Jan. 26, 1981 JP-A-57-124248 Combustion or D.sub.R Emissions engine exhaust Feb. 5, 1976 JP-A-52-95289 Combustion Response exhaust waveform TOYOTA MOTOR Feb. 9, 1979 JP-A-55-106353 Bottled Response Catalyst CORP gases waveform function, etc Nov. 10, 1977 JP-B-58-32655 Impedance TOYOTA MOTOR Nov. 17, 1977 JP-B-61-42225 Combustion Equilibrium .lambda. Control air/ CORP/TOYOTA exhaust fuel ratio CENTRAL RES & Oct. 15, 1977 JP-B-61-42224 Bottled D.sub.R DEV LAB INC gases __________________________________________________________________________
Sensors that use either a zirconia oxygen concentration cell or a titanium dioxide resistor are widely used as stoichiometric air/fuel ratio (O.sub.2) sensors for this three-way catalyst type of exhaust purifying system.
If a zirconia oxygen concentration cell type of stoichiometric air/fuel ratio (O.sub.2) sensor is used, output is in the form of an electromotive force, so that rich or lean can be determined from comparing the electromotive force of the sensor with a reference voltage. A constant voltage (usually 0.45 V) corresponding to the electromotive force at the stoichiometric air/fuel ratio is used as this reference voltage. A zirconia oxygen concentration cell type of stoichiometric air/fuel ratio (O.sub.2) sensor has extremely good characteristics in that the electromotive force has a low dependency on temperature, which means that there is no need to adjust the reference voltage for temperature, making it extremely easy to use, and thus it is the most widely used type of sensor.
With a titanium dioxide resistor type of stoichiometric air/fuel ratio (O.sub.2) sensor, output is in the form of a resistance, so that rich or lean can be determined from comparing the resistance of the sensor with a reference resistance. A constant resistance corresponding to the resistance at the stoichiometric air/fuel ratio is used as the reference resistance, but the sensor resistance is highly dependent on temperature, so that it is necessary to adjust the reference resistance for temperature when the sensor is used over a wide temperature range. This means that the air/fuel ratio control system may have to use a method of automatically adjusting the reference resistance.
Regardless of whether the stoichiometric air/fuel ratio (O.sub.2) sensor is a zirconia oxygen concentration cell type or a titanium dioxide resistor type, it is capable of operating at temperatures of approximately 400.degree. C. or above. And thus a characteristic that is ideal for detecting a stoichiometric air/fuel ratio with an error on the order of 1% can be obtained by selecting sensors of quality after fabrication. Therefore, both types of sensor can function at their best in a three-way catalyst type of exhaust purifying system, which will help immensely in reducing pollution in Earth's environment.
However, recent increases in the numbers of automobiles in use and in the weights of the vehicles themselves have led to a great deal of public concern relating to a greater reduction in the quantity of harmful components discharged into the atmosphere, while the trend toward reducing these harmful components has become dull. To address this public concern, not only is it necessary to improve catalysts and engines themselves, but it has become even more important to improve stoichiometric air/fuel ratio (O.sub.2) sensors. In other words, stoichiometric air/fuel ratio (O.sub.2) sensors must now be able to achieve a higher accuracy of air/fuel ratio detection than that of conventional products.
To provide a stoichiometric air/fuel ratio (O.sub.2) sensor of a quality that greatly exceeds that of conventional products, it is obviously necessary that improvements to the art should start at the design stage and extend through the entire fabrication process. But this does not mean that the above concerns will be addresses by design and fabrication improvements alone. Regardless of how the design and fabrication process are improved, product quality cannot be verified without a means of precisely measuring the accuracy with which the air/fuel ratio of the resultant stoichiometric air/fuel ratio (O.sub.2) sensor is detected. If product quality cannot be verified, the effects of design and fabrication improvements cannot be verified either, and thus it is clear that proof of such improvements cannot be obtained.
In other words, a key point in the development and supply of a high-quality stoichiometric air/fuel ratio (O.sub.2) sensor is a precise means of measuring the air/fuel ratio detection accuracy thereof. To test a stoichiometric air/fuel ratio (O.sub.2) sensor for such a higher level of accuracy, it is essential to use a testing device (characteristic-analyzing apparatus) that has such a higher level of accuracy itself.
As stated previously, a prior-art apparatus for analyzing air/fuel ratio (O.sub.2) sensor characteristics is capable of testing a stoichiometric air/fuel ratio (O.sub.2) sensor for an accuracy on the order of 1%. However, an apparatus for analyzing air/fuel ratio (O.sub.2) sensor characteristics that has an even higher accuracy has not yet been invented.